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116           MOBPHOLOGY AND EVOLUTION
bony lamina  which sometimes roofs  in  the   superior petrosal sinus.
The cerebral convolutions do not impress the cranial bones deeply, as in some of the Gibbons.
The cranial bones are excavated by sinuses, which help to lighten the skull. The sphenoidal sinus is almost entirely contained within the pre-sphenoid, and is subdivided by a longitudinal septum. There is no inastoid antrum, but tympanic air cells are contained in the squamosal. There is no proper frontal air sinus on either side, but the bone is cancellous. The maxillsB are excavated by the antra of Highmore. So it is evident that the air spaces in the Orang are much fewer than those in Man ; and it will be seen later that they are less extensive than in the African Simiidse, The latter have well-marked frontal sinuses, and the sphenoidal sinuses are more extensive.
The septum nasi is composed of the same elements as in Man, but the turbinate bones are less complex than in him and in the African Simiidse.
The mandible  is an exceedingly large  and strong bone; but it shows many variations, which are of an individual  character  apart from the differences due to sex and age.    The broad ascending ramus has a shallow sigmoid notch, and the point of the coronoid process is slightly recurved; its height varies, but it always rises more above the body of the bone than in the Gibbons. The lower border  of the body is  straight, curved or sinuous; and the strong, dense symphysis slopes downwards and backwards.    On the back of the symphysis there is a pit for the genial muscles, and the halves of the mandible are united below it by a bony ledge called the " simian shelf."